This invention relates to the stabilization of gasoline mixtures using antioxidants. In one aspect, the invention relates to the stabilization of pyrolysis gasoline. In still another aspect, the invention relates to the stabilization of pyrolysis gasoline using a primary antioxidant and a secondary antioxidant.
Pyrolysis gasoline (Pygas) is produced by the steam cracking of heavy naphthas or gas oils to produce ethylene with Pygas being the byproduct of this process. The Pygas is high in unsaturated hydrocarbons and is used as a feedstock for petrochemical processes or is blended to produce motor gasoline. Because of the presence of the unsaturated hydrocarbons, the tendency of the Pygas to foul is high. Oxygen contamination leads to the formation of intermediate peroxides and this in combination with the unsaturated hydrocarbons produces high molecular weight polymeric gums.
It is common practice to add to polymers, petroleum products, and food, antioxidants to retard oxidation. The two most common classes of antioxidants are the primary antioxidants and secondary antioxidants. The primary antioxidants include the phenylenediamines and hindered phenols and the secondary antioxidants generally include the phosphites and on infrequent occasions sulphur compounds such as thioethers and esters of thiodipropionic acid. The combination of antioxidants imparts multifunctionality to the treatment. The primary antioxidant functions as a free-radical scavenger, and the secondary antioxidants decompose hydroperoxides.
Antioxidants have been used in stabilizing gasoline and hydrocarbon streams. Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 3, page 437, 3rd edition, discloses the use of butylphenols, cresols, and phenylenediamines as fuel and lubricant stabilizers. U.S. Pat. No. 4,469,586 discloses the use of saturated sulfoxide in a hydrocarbon stream to reduce fouling in heat exchangers. U.S. Pat. No. 4,981,495 discloses the use of an alkyl 1,2-dihydroquinoline compound along with a hindered phenol in gasoline mixtures. U.S. Pat. No. 4,744,881 discloses the use of nonhindered or partially hindered phenols in combination with a strongly basic material such as an organo amine for use in pyrolysis gasoline and other hydrocarbons. U.S. Pat. No. 5,169,410 discloses the use of phenylenediamines in combination with a strongly basic organic amine in the stabilization of gasoline.
As noted above, the secondary antioxidants used in a variety of applications include the phosphites and certain sulfur compounds such as sulfides. For environmental and operational reasons, discussed below, these compounds cannot be used with gasolines. Phosphites create undesirable side effects such as a decrease in the reactor catalyst activity by phosphorus poisoning. Moreover, phosphites are disallowed in finished gasoline. The sulfur compounds have not been used in the treatment of gasoline because of concern with exceeding the EPA limit of 50 ppm of sulfur in the gasoline.
Thus, there is a need for inexpensive and effective secondary antioxidants that can be used with primary antioxidants to stabilize Pygas and other gasolines containing unsaturated hydrocarbons (e.g. olefins).